The aim is to vary certain male characteristics and to observe the result of this variation in the females. For the canary, my methods mainly involve certain song characteristics. With regard to the partridges, which are not songbirds, the varying characteristic will be the fact that the male partners are chosen or imposed on the female. The effect of these variations will be quantified with data collected on eggs laid by females (quantity, weight, concentration of testosterone...). All these data are ways of determining the investment of the female in the brood.